Plainfield is a City in Union County, New Jersey, United States. As of the United States 2000 Census, the city population was 47,829.

Plainfield was originally formed as a township on April 5, 1847, from portions of Westfield Township, while the area was still part of Essex County. On March 19, 1857, it became part of the newly-created Union County. Plainfield was incorporated as a city by an Act of the New Jersey Legislature on April 21, 1869, from portions of Plainfield Township, based on the results of a referendum held that same day. The city and township coexisted until March 6, 1878, when Plainfield Township was dissolved and parts absorbed by Plainfield City and the remainder becoming Fanwood Township (now known as Scotch Plains). "The Story of New Jersey's Civil Boundaries: 1606-1968", John P. Snyder, Bureau of Geology and Topography; Trenton, New Jersey; 1969. p. 240.

Plainfield is nicknamed "The Queen City".